Does the calculator work on a 30/360 basis, or actual/365, or actual/actual?
Strictly speaking, none of the above. For all payment schedules, the calculator treats the “Payments per Year” as equally-distributed. E.g., a “bi-weekly” payment schedule, 26 payments per year, is treated as having 365/26=14.038… days per payment period by the calculator. A true bi-weekly schedule would instead use 14 days exactly, and every 10 years, there would be an extra (27th) payment during the year. A 30/360 basis treats the year as having 360 days, each month having 30 days, resulting in 12 equally-spaced payments per year. For monthly payments, the calculator appears to be on a 30/360 basis since the math ends up the same: (APR×30)/360 is the same as APR/12. My understanding of the “actual” bases is that the APR is divided by the number of days in the year to provide a daily interest rate, and the interest is calculated on the true number days between scheduled payments. If one were to plot the interest paid over time, the actual basis methods will produce slightly jittery
